A wiring substrate in which a low-density wiring layer including an insulating layer famed of a thermosetting resin is formed on a core substrate and a high-density wiring layer including an insulating layer formed of a photosensitive resin is formed on the low-density wiring layer is proposed. (See, for example, Japanese Laid-open Patent Publication No. 2014-225632.) In recent years, electronic components have been reduced in size to be mounted on wiring substrates at a higher density. As a result, there is an increasing demand for reduction in the size and thickness and an increase in the signal transmission speed of wiring substrates.